Properties of alkali metal compounds

In this experiment, students use solutions of a range of alkali metal compounds to investigate their different properties. They record the colour of the compounds, check their solubility and pH and use their observational skills to complete flame tests for each solution.

This practical is suitable for pre-16 students. Most of the experiment can be done as a class practical. It takes about 20 minutes for students to investigate around four compounds.

Students should NOT be allowed to handle solid alkali metal hydroxides. You should demonstrate their properties.

You should decide which method you want to use for the flame tests, and if using an alternative method to that which is given here, carry out an appropriate risk assessment.

You need to decide in advance what strategy to use in allocating compounds to pairs of students.

A student worksheet with instructions and questions is available for download below as a PDF or editable Word document.

Equipment

Apparatus

• Eye protection (including goggles for teacher)
• Test tubes in a rack, x2 or 3
• Rubber bung to fit test tube
• Glass rod
• White tile
• Universal indicator paper and a pH colour chart
• Wooden splints pre-soaked in distilled water, for flame tests (see note 8 below)
• Bunsen burner
• Heat resistant mat

Chemicals

• Purified water
• A range of solid alkali metal compounds such as:
  • Sodium and potassium nitrates (OXIDISING)
  • Sodium and potassium chlorides
  • Sodium and potassium sulfates
  • Lithium, sodium and potassium carbonates (IRRIANT)
  • Sodium and potassium hydroxides (CORROSIVE) – for teacher demonstration only

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout.
• Sodium nitrate, NaNO₃(s), (OXIDISING, HARMFUL) and potassium nitrate, KNO₃(s), (OXIDISING) – see CLEAPSS Hazcard HC093. Alkali metal nitrates are deliquescent (absorb moisture from the air) and must be kept in stoppered bottles until shortly before use.
• Sodium chloride, NaCl(s), and potassium chloride, KCl(s) – see CLEAPSS Hazcard HC047b and CLEAPSS Recipe Book RB082 and RB068.  
• Sodium sulfate, Na₂SO₄(s), and potassium sulfate, K₂SO₄(s) – see CLEAPSS Hazcard HC098B. 
• Lithium carbonate, Li₂CO₃(s), (HARMFUL), sodium carbonate, Na₂CO₃(s), (IRRITANT) and potassium carbonate, K₂CO₃(s), (IRRITANT) – see CLEAPSS Hazcards HC058b and HC095A, plus CLEAPSS Recipe Book RB080. 
• Sodium hydroxide, NaOH(s), (CORROSIVE) and potassium hydroxide, KOH(s), (CORROSIVE) – see CLEAPSS Hazcards HC091a and HC091b. Alkali metal hydroxides react with the carbon dioxide in the air and must be kept in stoppered bottles until shortly before use. 
• In advance of the lesson, a supply of wooden splints need to be thoroughly soaked in distilled water.

Procedure

For teacher demonstration

Students should not dissolve solid sodium hydroxide or potassium hydroxide themselves. However, it is useful for students to see the precautions necessary when dissolving a compound with a very large negative enthalpy of solution. Wearing goggles (not safety spectacles), demonstrate the behaviour of the solid hydroxides by adding a single pellet to water, in a test tube or small beaker. Point out the large rise in temperature. Small samples of the solution can then be used for the pH test. Sodium hydroxide solutions are IRRITANT at or above 0.05 M and CORROSIVE at or above 0.5 M.

For student experiments

1. Note the name of the compound and find out whether any special precautions are necessary when handling it. (Students should be told which reference book to use and should check with their teacher before proceeding.)
2. Note the colour of the compound.
3. Shake a small amount of the compound (a few crystals) in a test tube with about 2 cm depth of purified water. Note whether the solid dissolves, and whether the test tube gets warm.
4. Use a glass rod to put a drop of the solution on to a small piece of universal indicator paper, and compare the colour with a chart to find the pH.
5. Take a wooden splint, pre-soaked in distilled water, and place it in the solution in the test tube. Allow some of the solution to soak into the splint then hold the splint in the side of a roaring Bunsen flame. Record the flame colour. Look up the formula of the compound.
6. Record all the results from their group in a table with suitable headings, such as these:
   • Name of compound
   • Hazards
   • Colour
   • pH
   • Test for solubility observation
   • Flame test; colour
   • Formula

Teaching notes

When testing pH the students should compare their solutions with the effect of the purified water which is unlikely to be neutral (distilled water will be weakly acidic, due to dissolved CO₂). Only if there is a difference can they suggest that the alkali metal compound is affecting the pH.

This activity provides an opportunity to explore the common misconception among students that alkali metal compounds are alkaline because of the presence of the alkali metal ions. If this were so, all the compounds would be alkaline in solution and not just the hydroxides and carbonates.

The accompanying student worksheet is available for download below.

Notes on the results

All the compounds suggested are colourless or white. The compounds are soluble in water. The hydroxides and carbonates are alkaline. Flame colours: lithium – bright red; sodium – bright yellow/orange; potassium – mauve/lilac (often masked by traces of sodium compounds in the sample).